Directional display apparatus

Abstract

A switchable privacy display comprises a spatial light modulator (SLM), a first switchable liquid crystal (LC) retarder and first passive retarder between a first pair of polarisers and a second switchable LC retarder and second passive retarder between a second pair of polarisers. The first switchable LC retarder comprises two homeotropic alignment layers and the second switchable LC retarder comprises two homogeneous alignment layers. In privacy mode, on-axis light from the SLM is directed without loss, whereas off-axis light has reduced luminance to reduce visibility to off-axis snoopers. The display may achieve privacy operation in landscape and portrait orientations. Further, display reflectivity may be reduced for on-axis reflections of ambient light, while reflectivity may be increased for off-axis light to achieve increased visual security. In public mode, the LC retardance is adjusted so that off-axis luminance and reflectivity are unmodified. The display may switch between day-time and night-time operation.

The invention claimed is: 
     
       1. A display device comprising:
 a spatial light modulator; 
 a display polariser arranged on a side of the spatial light modulator, the display polariser being a linear polariser; 
 a first additional polariser arranged on the same side of the spatial light modulator as the display polariser, the first additional polariser being a linear polariser; 
 at least one first polar control retarder arranged between the first additional polariser and the display polariser; 
 a second additional polariser, the second additional polariser being a linear polariser; and 
 at least one second polar control retarder, wherein either:
 the second additional polariser is arranged on the same side of the spatial light modulator as the first additional polariser outside the first additional polariser, and the at least one second polar control retarder is arranged between the first additional polariser and the second additional polariser; or 
 the display device further comprises a backlight arranged to output light, the spatial light modulator comprises a transmissive spatial light modulator arranged to receive output light from the backlight, said display polariser is an input display polariser arranged on the input side of the spatial light modulator, and the display device further comprises an output display polariser arranged on the output side of the spatial light modulator, the second additional polariser is arranged on the output side of the spatial light modulator, and the at least one second polar control retarder is arranged between the second additional polariser and the output display polariser, 
 
 wherein: 
 each of the at least one first polar control retarder and the at least one second polar control retarder comprises a respective switchable liquid crystal retarder comprising a layer of liquid crystal material and two surface alignment layers disposed adjacent to the layer of liquid crystal material and on opposite sides thereof, 
 in respect of one of the at least one first polar control retarder and the at least one second polar control retarder, both of the surface alignment layers are arranged to provide homogenous alignment in the adjacent liquid crystal material, and 
 in respect of the other of the at least one first polar control retarder and the at least one second polar control retarder, both of the surface alignment layers are arranged to provide homeotropic alignment in the adjacent liquid crystal material. 
 
     
     
       2. A display device according to  claim 1 , wherein
 the display device further comprises a reflective polariser, the reflective polariser being a linear polariser, and either:
 said display polariser is an output display polariser arranged on the output side of the spatial light modulator, the second additional polariser is arranged on the same side of the spatial light modulator as the first additional polariser outside the first additional polariser, the at least one second polar control retarder is arranged between the first additional polariser and the second additional polariser, and the reflective polariser is arranged between the first additional polariser and the at least one second polar control retarder; or 
 the display device further comprises a backlight arranged to output light, the spatial light modulator comprises a transmissive spatial light modulator arranged to receive output light from the backlight, said display polariser is an input display polariser arranged on the input side of the spatial light modulator, and the display device further comprises an output display polariser arranged on the output side of the spatial light modulator, the second additional polariser is arranged on the output side of the spatial light modulator, the at least one second polar control retarder is arranged between the second additional polariser and the output display polariser, and the reflective polariser is arranged between the output display polariser and at least one second polar control retarder. 
 
 
     
     
       3. A display device according to  claim 2 , wherein
 the switchable liquid crystal retarder of said one of the at least one first polar control retarder and the at least one second polar control retarder has a retardance for light of a wavelength of 550 nm having a first retardance value and the switchable liquid crystal retarder of said other of the at least one first polar control retarder and the at least one second polar control retarder has a retardance for light of a wavelength of 550 nm having a second retardance value, and either: 
 the first retardance value is greater than the second retardance value and the magnitude of the difference between half the first retardance value and the second retardance value is at most 400 nm; or 
 the first retardance value is less than the second retardance value and the magnitude of the difference between the first retardance value and half the second retardance value is at most 400 nm. 
 
     
     
       4. A display device according to  claim 3 , wherein the first retardance value is greater than the second retardance value and the magnitude of the difference between half the first retardance value and the second retardance value is at most 400 nm. 
     
     
       5. A display device according to  claim 4 , wherein
 the first retardance value is in a range from 700 nm to 2500 nm, and 
 said one of the at least one first polar control retarder and the at least one second polar control retarder further comprises a pair of passive uniaxial retarders having optical axes in the plane of the retarders that are crossed and each having a retardance for light of a wavelength of 550 nm in a range from 600 nm to 1600 nm, 
 the second retardance value is in a range from 500 nm to 900 nm, and 
 said other of the at least one first polar control retarder and the at least one second polar control retarder further comprises a passive uniaxial retarder having its optical axis perpendicular to the plane of the retarder and having a retardance for light of a wavelength of 550 nm in a range from −300 nm to −800 nm. 
 
     
     
       6. A display device according to  claim 3 , wherein the first retardance value is less than the second retardance value and the magnitude of the difference between the first retardance value and half the second retardance value is at most 400 nm. 
     
     
       7. A display device according to  claim 6 , wherein
 the first retardance value is in a range from 450 nm to 900 nm, and 
 said one of the at least one first polar control retarder and the at least one second polar control retarder further comprises a pair of passive uniaxial retarders having optical axes in the plane of the retarders that are crossed and each having a retardance for light of a wavelength of 550 nm in a range from 250 nm to 800 nm, 
 the second retardance value is in a range from 700 nm to 2500 nm, and 
 said other of the at least one first polar control retarder and the at least one second polar control retarder further comprises a passive uniaxial retarder having its optical axis perpendicular to the plane of the retarder and having a retardance for light of a wavelength of 550 nm in a range from −700 nm to −2500 nm. 
 
     
     
       8. A display device according to  claim 6 , wherein
 the first retardance value is in a range from 500 nm to 900 nm, and 
 said one of the at least one first polar control retarder and the at least one second polar control retarder further comprises a passive uniaxial retarder having its optical axis perpendicular to the plane of the retarder and having a retardance for light of a wavelength of 550 nm in a range from −300 nm to −700 nm, 
 the second retardance value is in a range from 700 nm to 2500 nm, and 
 said other of the at least one first polar control retarder and the at least one second polar control retarder further comprises a passive uniaxial retarder having its optical axis perpendicular to the plane of the retarder and having a retardance for light of a wavelength of 550 nm in a range from −700 nm to −2500 nm. 
 
     
     
       9. A display device according to  claim 3 , wherein the retardance of the switchable liquid crystal retarder of the at least one first polar control retarder is less than the retardance of the switchable liquid crystal retarder of the at least one second polar control retarder. 
     
     
       10. A display device according to  claim 1 , wherein
 the display device does not comprise a reflective polariser, and 
 the switchable liquid crystal retarder of said one of the at least one first polar control retarder and the at least one second polar control retarder has a retardance for light of a wavelength of 550 nm having a first retardance value and the switchable liquid crystal retarder of said other of the at least one first polar control retarder and the at least one second polar control retarder has a retardance for light of a wavelength of 550 nm having a second retardance value, the magnitude of the difference between the first retardance value and the second retardance value is at most 400 nm. 
 
     
     
       11. A display device according to  claim 10 , wherein said one of the at least one first polar control retarder and the at least one second polar control retarder further comprises a pair of passive uniaxial retarders having optical axes in the plane of the retarders that are crossed. 
     
     
       12. A display device according to  claim 11 , wherein
 the first retardance value is in a range from 500 nm to 900 nm, and 
 said pair of passive uniaxial retarders of said one of the at least one first polar control retarder and the at least one second polar control retarder each have a retardance for light of a wavelength of 550 nm in a range from −300 nm to −700 nm. 
 
     
     
       13. A display device according to  claim 10 , wherein said other of the at least one first polar control retarder and the at least one second polar control retarder further comprises a passive uniaxial retarder having its optical axis perpendicular to the plane of the retarder. 
     
     
       14. A display device according to  claim 13 , wherein
 the second retardance value is in a range from 500 nm to 900 nm, and 
 said passive uniaxial retarder of said other of the at least one first polar control retarder and the at least one second polar control retarder has a retardance for light of a wavelength of 550 nm in a range from −300 nm to −700 nm. 
 
     
     
       15. A display device according to  claim 1 , wherein
 said surface alignment layers of said one of the at least one first polar control retarder and the at least one second polar control retarder have pretilts having pretilt directions with components in the plane of the layer of liquid crystal material in a first pair of anti-parallel directions, and 
 said surface alignment layers of said other of the at least one first polar control retarder and the at least one second polar control retarder have pretilts having pretilt directions with components in the plane of the layer of liquid crystal material in a second pair of anti-parallel directions, the first pair of anti-parallel directions being crossed with the second pair of anti-parallel directions. 
 
     
     
       16. A display device according to  claim 15 , wherein the first pair of anti-parallel directions are at 90 degrees to the second pair of anti-parallel directions, as viewed normal to the planes of the layers of liquid crystal material of the at least one first polar control retarder and the at least one second polar control retarder. 
     
     
       17. A display device according to  claim 15 , wherein the switchable liquid crystal retarder of said one of the at least one first polar control retarder and the at least one second polar control retarder has a retardance for light of a wavelength of 550 nm having a first retardance value and the switchable liquid crystal retarder of said other of the at least one first polar control retarder and the at least one second polar control retarder has a retardance for light of a wavelength of 550 nm having a second retardance value, the magnitude of the difference between the first retardance value and the second retardance value is at most 400 nm. 
     
     
       18. A display device according to  claim 17 , wherein said one of the at least one first polar control retarder and the at least one second polar control retarder further comprises a pair of passive uniaxial retarders having optical axes in the plane of the retarders that are crossed. 
     
     
       19. A display device according to  claim 18 , wherein
 the first retardance value is in a range from 500 nm to 900 nm, and 
 said pair of passive uniaxial retarders of said one of the at least one first polar control retarder and the at least one second polar control retarder each have a retardance for light of a wavelength of 550 nm in a range from −300 nm to −700 nm. 
 
     
     
       20. A display device according to  claim 17 , wherein said other of the at least one first polar control retarder and the at least one second polar control retarder further comprises a passive uniaxial retarder having its optical axis perpendicular to the plane of the retarder. 
     
     
       21. A display device according to  claim 20 , wherein
 the second retardance value is in a range from 500 nm to 900 nm, and 
 said passive uniaxial retarder of said other of the at least one first polar control retarder and the at least one second polar control retarder has a retardance for light of a wavelength of 550 nm in a range from −300 nm to −700 nm. 
 
     
     
       22. A display device according to  claim 15 , wherein the display device further comprises a reflective polariser, the reflective polariser being a linear polariser,
 said display polariser is an output display polariser arranged on the output side of the spatial light modulator, the second additional polariser is arranged on the same side of the spatial light modulator as the first additional polariser outside the first additional polariser, the at least one second polar control retarder is arranged between the first additional polariser and the second additional polariser, and the reflective polariser is arranged between the first additional polariser and the at least one second polar control retarder; 
 
       wherein
 the spatial light modulator comprises an emissive spatial light modulator arranged to output light, the emissive spatial light modulator comprising an array of pixels arranged in a pixel layer, 
 the display polariser is an output display polariser arranged on the output side of the emissive spatial light modulator, the second additional polariser is arranged on the output side of the spatial light modulator outside the first additional polariser, and the at least one second polar control retarder is arranged between the first additional polariser and the second additional polariser, 
 the display device further comprises a parallax barrier forming an array of apertures, wherein the parallax barrier is separated from the pixel layer by a parallax distance along an axis along a normal to the plane of the pixel layer, each pixel being aligned with an aperture, and 
 the retardance of the switchable liquid crystal retarder of the at least one second polar control retarder is less than the retardance of the switchable liquid crystal retarder of the at least one first polar control retarder. 
 
     
     
       23. A display device according to  claim 22 , wherein
 said one of the at least one first polar control retarder and the at least one second polar control retarder is the at least one second polar control retarder, 
 the switchable liquid crystal retarder of the at least one second polar control retarder has a retardance for light of a wavelength of 550 nm in a range from 200 nm to 550 nm, 
 the at least one second polar control retarder further comprises a pair of passive uniaxial retarders having optical axes in the plane of the retarders that are crossed and each having a retardance for light of a wavelength of 550 nm in a range from 150 nm to 550 nm, 
 said other of the at least one first polar control retarder and the at least one second polar control retarder is the at least one first polar control retarder, 
 the switchable liquid crystal retarder of the at least one first polar control retarder has a retardance for light of a wavelength of 550 nm in a range from 500 nm to 900 nm, and 
 the at least one first polar control retarder further comprises a passive uniaxial retarder having its optical axis perpendicular to the plane of the retarder and having a retardance for light of a wavelength of 550 nm in a range from −300 nm to −1000 nm. 
 
     
     
       24. A display device according to  claim 1 , wherein the spatial light modulator comprises an emissive spatial light modulator arranged to output light, the display polariser is an output display polariser arranged on the output side of the emissive spatial light modulator, the second additional polariser is arranged on the output side of the spatial light modulator outside the first additional polariser, and the at least one second polar control retarder is arranged between the first additional polariser and the second additional polariser. 
     
     
       25. A display device according to  claim 24 , wherein
 the emissive spatial light modulator comprises an array of pixels arranged in a pixel layer, and 
 the display device further comprises a parallax barrier forming an array of apertures, wherein the parallax barrier is separated from the pixel layer by a parallax distance along an axis along a normal to the plane of the pixel layer, each pixel being aligned with an aperture. 
 
     
     
       26. A display device according to  claim 22 , wherein the emissive spatial light modulator has an output luminance profile having a full width half maximum that is at most 40 degrees. 
     
     
       27. A display device according to  claim 1 , wherein the display device comprises a backlight arranged to output light, and the spatial light modulator comprises a transmissive spatial light modulator arranged to receive output light from the backlight. 
     
     
       28. A display device according to  claim 27 , wherein the backlight has an output luminance profile having a full width half maximum that is at most 40 degrees. 
     
     
       29. A display device according to  claim 27 , wherein:
 said display polariser is an input display polariser arranged on the input side of the spatial light modulator; 
 the first additional polariser is arranged between the backlight and the input display polariser; and 
 the second additional polariser is arranged on the same side of the spatial light modulator as the first additional polariser between the backlight and the first additional polariser, and the at least one second polar control retarder is arranged between the first additional polariser and the second additional polariser. 
 
     
     
       30. A display device according to  claim 1 , wherein each of the at least one first polar control retarder and the at least one second polar control retarder further comprises at least one passive compensation retarder. 
     
     
       31. A display device according to  claim 30 , wherein the at least one passive compensation retarder of the at least one first polar control retarder and the at least one second polar control retarder comprises either:
 a passive uniaxial retarder having its optical axis perpendicular to the plane of the retarder; or 
 a pair of passive uniaxial retarders having optical axes in the plane of the retarders that are crossed. 
 
     
     
       32. A display device according to  claim 1 , wherein said other of the at least one first polar control retarder and the at least one second polar control retarder is between the backlight and the transmissive spatial light modulator. 
     
     
       33. A display device according to  claim 1 , wherein said one of the at least one first polar control retarder and the at least one second polar control retarder is on the output side of said other of the at least one first polar control retarder and the at least one second polar control retarder.